Magnetic resonance apparatus

Abstract: In a magnetic resonance apparatus including an active noise reduction arrangement (9, 49, 51, 53) for use by a patient (5) undergoing examination in the apparatus the transducers (25, 31) of the arrangement are non-magnetic, e.g. are of the electret, piezoelectric or condenser type, and have no parts of ferromagnetic material.

Description: This invention relates to magnetic resonance apparatus. Such apparatus can be used for non-invasive internal examinations of patients to produce, for example, cross-sectional internal structural images, and blood flow and spectroscopy data. In use of such apparatus the part of the patient to be examined is placed in a region of strong static magnetic field to define an equilibrium axis of magnetic alignment in the examination region. A radio frequency (RF) magnetic field is then applied temporarily to the examination region, in a direction orthogonal to the static magnetic field direction, to excite magnetic resonance in material, typically hydrogen protons, in the examination region. The resulting RF signals are detected and analysed. During this sequence of operations one or more gradients are normally imposed on the static magnetic field in the examination region, e.g. to encode spatially the detected RF signals, or for other purposes such as flow encoding. The various fields applied in use of a magnetic resonance apparatus give rise to appreciable noise which can be disturbing to a patient, especially when the patient is very ill and/or the patient's head is being examined. To overcome this difficulty it has been proposed to use an active noise reduction arrangement. Known active noise reduction arrangements giving the required level of performance comprise a headset incorporating two enclosures which fit respectively around the two ears of the patient. Each enclosure houses a microphone which senses noise in the enclosure. The microphone signal is processed remote from the headset and a resulting signal applied to a loudspeaker in the enclosure to produce sound waves of a phase, amplitude and frequency such as to tend to reduce the noise heard by the patient to a minimum. One problem which arises with such an arrangement is that the headset interferes with the static and RF magnetic fields required in the examination region, more particularly the required uniformity of such fields. WO91/15896 discloses such a noise reduction system in which the microphone and loudspeakers are housed in ported enclosures which are located outside the RF field of the magnetic resonance apparatus to prevent them from interfering with magnetic resonance signals. Ports made of non-magnetic material are provided from the loudspeaker microphone enclosures for conveying the noise reduction acoustic signals to a region adjacent to the patients head. WO90/02513 also discloses an active noise reduction arrangement in which the loudspeaker and microphone are located outside the RF field and in which the noise reduction acoustic signals are conveyed by respective flexible tubes to a headset worn by the patient. To minimise interference with the magnetic resonance signals the headset and tubes are each made from non-magnetic materials. It is an object of the present invention to provide a magnetic resonance apparatus for examining a patient wherein the problem of interference is alleviated. According to the present invention there is provided a magnetic resonance apparatus including means arranged to emit an RF field of a predetermined frequency to excite a magnetic resonance in a patient to be examined and a noise reduction arrangement for use by the patient during examination, characterised in that the noise reduction arrangement comprises electrically-controlled non-ferromagnetic transducers which are disposed in a part of said RF field and have at least one lead connected thereto and wherein the lead is grounded to signals of the frequency of said RF field by way of a filter tuned to said RF frequency at a distance along the lead from the transducer to which it is connected equal to a quarter of a wavelength at said RF frequency or, to an odd multiple thereof. In a preferred embodiment of the invention the transducers are incorporated in a headset for wear by the patient, substantially all parts of which headset are formed of non-ferromagnetic material. In one such apparatus each said lead is grounded to a virtual earth. Said virtual earth may comprise an open circuit terminated line of length equal to a quarter of a wavelength at said RF field frequency or to an odd multiple thereof Advantageously each conductor and, if present a screen of said lead is separately grounded. In a preferred embodiment at least one of said transducers is a piezoelectric loudspeaker of the kind comprising a film of piezoelectric plastics film carrying an electrode made of a non-ferromagnetic electrically conducting material on each main surface and said film being elastically supported within an enclosure in a domed condition such that electrically expansion and contraction of the film is converted to a pistophonic motion. Preferably the electrode has a number of slots which divide the electrode into a plurality of electrically connected parallel strip portions, thereby to reduce the circulation of eddy currents in the electrode According to a further aspect of the invention, as defined in claim 9, an active noise reduction arrangement is adapted for use in a magnetic resonance apparatus as described above. One magnetic resonance apparatus in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which:- Figure 1 is a diagram illustrating a patient examination region of the apparatus. Figure 2 is a diagram illustrating an earpiece of an active noise reduction headset used in the apparatus. Figure 3 illustrates the form of an electrode of a transducer of the headset; and Figure 4 is a diagram illustrating the form of a lead to a transducer of the headset. The apparatus, which produces images for medical diagnostic purposes is, for the most, part of conventional form. Referring to Figure 1, the apparatus includes an electromagnet assembly 1 which defines a tubular bore 3 in which, in use of the apparatus, a patient 5 to be examined lies on a patient support 57. The assembly 1 comprises a main electromagnet which produces a strong, uniform, static magnetic field in the bore 3 in the direction of the axis of the bore, normally referred to as the z-direction. The assembly 1 further includes a gradient coil system whereby a gradient may be imposed on the static magnetic field in the bore 3 in any one or more of three orthogonal directions, i.e. x, y and z directions. The apparatus further includes an RF coil system 7 whereby RF field pulses may be applied to at least that part of a patient positioned in the bore 3 which it is desired to examine. The applied RF field pulses excite magnetic resonance in the part of the patient to be examined. The RF coil system 7 also serves to detect RF signals resulting from the excited magnetic resonance. In Figure 1 the RF coil system 7 is depicted, by way of example, as being of tubular form and disposed coaxial with the bore 1, surrounding the patient's head. Energisation of the electromagnet assembly 1 and RF coil system 7 and processing of the detected RF signals to produce an image is effected under the control of a computer (not shown) and associated power supply and control equipment (not shown). In operation of the apparatus the assembly 1 and the RF coil system 7 produce a significant amount of noise which can be upsetting to a patient, particularly in the confined space within the RF coil system 7. To reduce this noise to a minimum the patient 3 is provided with an active noise reduction (ANR) arrangement. The ANR arrangement includes a headset 9 having two earpieces 11 each in the form of an enclosure which fits around a respective ear of the patient 5. The two earpieces 11 are supported on a headband 13 of springy plastics material which serves to urge the earpieces 11 firmly against the patient's head. Referring to Figure 2, each earpiece 11 includes an outer shell 15 of a polymer-based material which may be loaded, for example, with a material such as glass, carbon, alumina or silica. Within the open end of the shell 15 there fits a plastics foam ring 17 to whose outer end is affixed a rigid flange plate 19 of plastics material, e.g. a polymer-based material, the plate 19 supporting a plastics foam cushion ring 21 which contacts the patient's head. The enclosure formed by the shell 15 and foam rings 17 and 21 is divided into two parts by a perforated polycarbonate grille 23 which is positioned on the side of the ring 17 remote from the flange plate 19. A microphone 25 is mounted in the outer part of the enclosure centrally on the outer side of the grille 23 so as to be spaced from the user's ear. Other positions for the microphone 25 may sometimes be preferable e.g towards one side of the grille 23, or under the flange plate 19, near the user's ear canal opening. The microphone 25 is protected from ingress of material from outside the earpiece 11 by an impermeable membrane 27. A permeable, second membrane 29 is provided outside the impermeable membrane 27 for hygiene reasons, and is changed for each patient. The two membranes 27 and 29 are attached at their outer peripheries to the flange plate 19 and may, if desired, be arranged to cover the cushion ring 21 for additional hygienic protection. The microphone 25 is non-magnetic, that is to say, it is of a kind which does not utilise a magnetic field for its operation, and is otherwise constructed using only non-ferromagnetic materials. To this end the microphone 25 is suitably of an electret type, but may alternatively be of a piezoelectric or condenser type. One suitable electret microphone is that described in the Journal of the Acoustic Society of America, Vol 40 (1966), part 6, pages 1433 to 1440, but with ferromagnetic materials removed or, where necessary, replaced by non-ferromagnetic material. In particular, the steel case is replaced by a brass case. The inner part of the earpiece enclosure houses a loudspeaker 31 which is also non-magnetic e.g. a condenser, electret or piezoelectric loudspeaker. In Figure 2 the loudspeaker 31 is depicted as a piezoelectric loudspeaker which has the advantages that it requires a relatively low drive voltage and no bias voltage. The loudspeaker 31 includes a film 33 of piezoelectric plastics material, for example polyvinylidene fluoride (PVDF), or a co-polymer of PVDF and trifluoroethylene, which is clamped around its periphery between two stiff polymer rings 35, suitably made of polycarbonate, the rings 35 being secured within the shell 15, and being provided with a by-pass hole 37, to equalise air pressure on the two sides of the film 33. The film 33 is provided on each side with an electrode 39 of non-ferromagnetic electrically conductive material, e.g. copper or aluminium. To reduce the possibility of eddy currents circulating in the electrodes 39, which can arise due to the rapid changes in the RF and gradient magnetic fields during an imaging sequence, each electrode 39 is divided by suitably positioned slots 41 into a series of narrow parallel strips 43, electrically connected together at one end, as illustrated in Figure 3. When an alternating potential is applied between the electrodes 39 of the film 33, the material of the film 33 expands and contracts in the plane of the film 33. Foam 45 placed behind the film 33 causes the film to dome, as shown in Figure 2. As a result of this, the expansion and contraction of the film material is converted to a pistophonic motion to generate sound waves directed towards the user's ear corresponding to the applied drive voltage. Electrical connection to the microphone 25 and loudspeaker 31 of each earpiece 11 is made by way of leads 47 (see Figure 1) which pass through the shell 15 via grommets (not shown), the leads 47 being in the form of a twisted pair of wires (not shown) of non-ferromagnetic metal surrounded by a braid screen (not shown) of non -ferromagnetic metal. It will be appreciated that by virtue of the complete absence of ferromagnetic material in the ANR headset 9, the presence of the headset does not significantly interfere with the homogeneity of the static magnetic and the RF fields in the examination region of the magnetic resonance apparatus. Hence, the presence of the headset 9 does not degrade images produced by the apparatus. In order to prevent image degradation by virtue of the headset loading the RF fields in the apparatus (as further explained below), the leads 47 connect the microphones 25 and the loudspeakers 31 to a signal processor 49 of the ANR arrangement by way of a filter unit 51 and virtual earth arrangement 53. The processor 49 utilises the signals produced by the microphones 25 to produce signals for application to the loudspeakers 31 of such a form as to minimise the noise heard by the patient 1. Thus the signals are required to be of such a form that the loudspeakers 31 produce at the patient's ears sound waves of the same amplitude and frequency content as, but of opposite phase to, the noise sound waves reaching the patient's ears. Referring to Figure 4, the filter unit 51 includes twelve series resonant circuit filters 55 tuned to the frequency of the RF fields applied to the examination region in use of the imaging apparatus, there being one filter 55 for each wire and the screen of each lead 47. Each filter 55 is connected between ground and a point on the associated wire or screen which is at a distance from the associated ANR headset transducer, i.e. microphone 25 or loudspeaker 31, equal to a quarter of a wavelength of waves of the RF field frequency propagating along the transducer lead 47. Each wire and screen is thus effectively grounded to RF field frequencies at this point and consequently presents a high impedance to currents at the RF field frequency at its end adjacent the associated ANR transducer 25 or 31. Hence the presence of the transducers 25 and 31 in the imaging apparatus examination region interferes with, i.e. loads the RF fields produced for imaging to a negligible extent. It will be understood that the passage of audio frequency signals along the leads 47 for noise reduction is not affected. Where, as is usually the case, a suitable ground connection is not available inside the bore 3 of the imaging apparatus (the bore 3 typically being internally lined with a plastics material cowling), the virtual earth 53 is required. This comprises a line of length equal to a quarter wavelength at the RF field frequency or to an odd multiple thereof, with an open circuit at its end remote from the associated filter 55. The line suitably comprises a serpentine track formed on one side of a plastics material sheet. The serpentine track may suitably be formed on the plastics material bore cowling, where possible. It will be understood that whilst in the ANR arrangement of the magnetic resonance apparatus described above, by way of example, the transducers are mounted in a headset worn by the patient, this is not necessarily the case. In other apparatuses according to the invention the ANR arrangement transducers may be otherwise supported, e.g. at appropriate places on the RF coil system 7, or other part of the magnetic resonance apparatus per se.

Claims: A magnetic resonance apparatus including means (7) arranged to emit an RF field of a predetermined frequency to excite a magnetic resonance in a patient (5) to be examined and a noise reduction arrangement for use by the patient (5) during examination, characterised in that the noise reduction arrangement comprises electrically-controlled non-ferromagnetic transducers (25,31) which are disposed in a part of said RF field and have at least one lead (47) connected thereto and wherein the lead (47) is grounded (51,53) to signals of the frequency of said RF field by way of a filter (55) tuned to said RF frequency at a distance along the lead from the transducer to which it is connected equal to a quarter of a wavelength at said RF frequency or, to an odd multiple thereof. An apparatus according to Claim 1 wherein the transducers are incorporated in a headset (9) for wear by the patient and wherein substantially all parts of said headset are formed of non-ferromagnetic material. An apparatus according to any preceding claim wherein each conductor and, if present, a screen of said lead (47) is separately grounded. An apparatus according to any preceding claim wherein said lead (47) is grounded to virtual earth (53). An apparatus according to Claim 4 wherein said virtual earth (53) comprises an open circuit terminated line of length equal to a quarter of a wavelength at said RF field frequency or to an odd multiple thereof. An apparatus according to any one of the preceding claims wherein each said transducer (25,31) is of an electret, condenser or piezoelectric type. An apparatus according to preceding claim wherein at least one (31) of said transducers (25,31) is a piezoelectric loudspeaker of the kind comprising a film of piezoelectric plastics film (33) carrying an electrode made of a non-ferromagnetic electrically conducting material (39) on each main surface and said film being elastically supported within an enclosure in a domed condition such that electrically expansion and contraction of the film is converted to a pistophonic motion An apparatus according to Claim 7 wherein the electrode (39) has a number of slots (41) which divide each electrode (39) into a plurality of electrically connected parallel strip portions (43), thereby to reduce the circulation of eddy currents in the electrode (39). An active noise reduction arrangement adapted for use in a magnetic resonance apparatus according to any one of the preceding claims, the arrangement comprising electrically-controlled non-ferromagnetic transducers (25,31) which have at least one lead (47) connected thereto and wherein the lead (47) is grounded to signals of a RF field of predetermined frequency by way of a filter (55) tuned to said RF frequency at a distance along the lead from the transducer to which it is connected equal to a quarter of a wavelength at said RF frequency or, to an odd multiple thereof.

Inventors: GILDERDALE DAVID JOHN, SALLOWAY ANTHONY JOHN, GILDERDALE, DAVID JOHN, SALLOWAY, ANTHONY JOHN
Applicants: MARCONI CASWELL LTD, MARCONI CASWELL LIMITED
Application year: 1994
Main classification: G01R33
IPC codes: G01R33, A61B5